When the speaker is an 8 Ω load, lets say all is well. But when the speaker drops to being a 3 Ω load, that will end up wanting to draw too much power from the amp/receiver and thus causing it to trip?
When an amplifier gets rated for power output, there must be an electrical load, an impedance, for it to drive. That load is considered the standard value for rating amps. Their power ratings always indicate an 8Ω load. Most or all amps now sold can handle loads of about 8Ω without trouble. Most can go below 8Ω without much trouble, but this depends on the speakers behavior. (More on this below.)
As the impedance load drops, the amp has to work harder to deliver power. With a 4Ω load, an amp has to work twice as hard. Some amps are robust enough to handle that. These amps often, but not always, come with power ratings for both 8Ω and 4Ω loads. In theory, such an amp can deliver as much as twice the power at 4Ω loads compared to 8Ω loads. In reality, this varies and is somewhat less the twice the power.
Other lower priced amps may not have a published 4Ω power rating, but under most circumstances, they can handle it. Other amps cannot. They can go into something called oscillation, a type of electrical misbehavior that can easily lead to amp failure. Most amps and receivers sold today have built-in protection systems that rapidly shut them down if oscillation is detected.
Now, when you say the minimum impedance is 3.1 Ω, does that mean that even though the speaker is simply rated as 8 Ω, that in reality there's a range (min to max)?
Below is a graph of a B&W 702 S2 speaker's Impedance (in Ω, solid black line) and Impedance Phase angle (in degrees, dotted black line). The vertical axis on the left side shows Ωs for Impedance, and the vertical axis on the right side shows the phase angle in degrees, from +90° to -90°. Both lines are plotted against frequency of the signal in Hz (on the horizontal axis).
I found this graph in review published by Stereophile. I couldn't find a similar graph for the more recent B&W 702 S3 speaker.
https://www.stereophile.com/content/bowers-wilkins-702-s2-loudspeaker-measurements
As you can see the Impedance (solid black line) is not a constant value – it varies with the frequency. We don't need to worry over the maximums, but are interested in knowing about the minimum impedance values.
- What frequencies are associated with the minimum impedance values?
- What are the impedance phase angles at those frequencies?
In the 702 S2 speaker, you can see two minimums below 4Ω. One is a bit above 100Hz, and the other is between 500 to 900 Hz, centered at about 700Hz. At the first minimum, the Phase Angle is
quite low, roughly -22.5°. But more importantly, the Phase Angle is lowest at 70Hz, and rapidly rises above that to as high as + angles at 200Hz. The text from the Stereophile page says this:
"The 702 S2's nominal impedance is specified as 8 ohms, with a minimum value of 3.1 ohms. My measurement of the impedance magnitude (fig.1, solid trace) reveals that while the impedance does lie at and above 8 ohms in the low treble and in two regions in the bass, it actually drops below 6 ohms through much the audioband, with a minimum value of 3 ohms in the upper bass. There is also a current-hungry combination of 4 ohms and a –48° electrical phase angle at 88Hz. Although technically this is an 8 ohm design, I think it should be used with amplifiers that are comfortable with 4 ohm loads."
The first low impedance value, at about 125Hz, combined with the low impedance phase angle, is likely responsible for this speakers requirement of a more robust amp.
